This invention relates to a vehicle traction control system.
When driving torque is applied to a wheel of a vehicle, a longitudinal driving force is generated between the driven wheel and the road surface that is dependent upon various parameters including the road surface conditions and the amount of spin between the driven wheel and the road surface. As the value of spin increases from zero, the longitudinal driving force increases to a peak value at a critical spin value. As spin is increased beyond this value, the longitudinal driving forces decrease. Therefore, at the critical spin value, the maximum possible vehicle acceleration results. If the vehicle operator initiated engine output results in the wheel spin exceeding the critical spin the vehicle acceleration decreases from the maximum possible acceleration for the road surface condition.
It is well known to sense this excessive spin condition and limit wheel spin in the proximity of the critical spin so as to maximize vehicle longitudinal acceleration. This is accomplished by limiting the driving torque delivered to the wheels such as by applying the vehicle brakes and/or limiting the engine output torque.
It is also known that the lateral force between the vehicle wheels and the road surface is at a maximum at zero wheel spin and decreases therefrom as spin increases from zero. The result is that maximum cornering or turning capability of a vehicle is maximum at zero wheel spin and decreases therefrom as spin increases.
Accordingly, limiting wheel spin at the value producing maximum vehicle acceleration necessarily results in a decrease in the lateral force between the wheel and the road surface. In order to maximize vehicle acceleration while traveling in a straight line and to assure vehicle stability while traveling through a curved path, it has been proposed to provide a traction controller that limits wheel acceleration spin at a value producing approximately the maximum longitudinal driving force while the vehicle is traveling over a straight path and provide for more aggressive control by limiting acceleration spin at a lower spin value when the vehicle is traveling over a curved path. It has also been suggested that the reduction in the spin limit be based upon the curvature of the path as represented by, for example, steering wheel angle.